How, well, that was the nature of the magic. You take on the form of a monster and gain the related size/mobility, but you don't gain any attack power. Thankfully for him, he already had the attack power to spare, and the size was all he needed. (I guess it'd count as "Illusion Magic" because you don't really become a monster; you just take the monster's form. It's not like a "mirage", though.)

Still, we have Leafa saying how crappy illusion is in battle every episode but Kirito uses it to turn into a giant monster and owns. Not to mention, Kirito got all of his skill levels from SAO which did not have magic (i.e. his illusion should be at a low level, unless a sword skill or something transferred to illusion, but that seems unlikely). So what then, is giant monster illusion your weakest spell? I smell an Ass Pull here